406 
THE TERTIARY VOLCANOES 
BOOK VIII 
it yields along its system ot joints to the influence of a tempestuous climate. 
It lias been carved into huge projecting buttresses and deep alcoves, the 
naked stone glowing with tints of orange and fawn colour, veiled here and 
there with patches of bright green slope, or edged with fringes of sea-pink 
and camomile. Every outstanding bastion is rent with chasms and split into 
blocks, which accumulate on the ledges like piles of ruined walls. To one 
who boats underneath these cliffs the scene of ceaseless destruction which 
they present is vividly impressive. 
Tire geology of St. Hilda was sketched by Macculloch, who recognized 
the close resemblance of its two groups of rock to the “ augite-roek ” (gabbro) 
and “ syenite ” (granophyre) of Skye and other islands of the Inner Hebrides. 
But he left the relations of the two groups to each other undetermined. 1 
Professor Heddle has published a brief reference to the rocks of St. Kilda, 
without, however, offering any definite opinion as to the geological structure 
ol the islands. 2 The best account of the geology has been given by Mr. 
Alexander Boss, who obtained evidence that the acid sends veins into the 
basic rock. He brought away specimens clearly showing this relation, but 
in his description left the question open for further inquiry. 8 To some of 
the observations in these papers reference will be made in the sequel. The 
following account is based on the results of two visits paid by me to St. 
Kilda in the summers of 1895 and 1896, during which I was enabled to 
examine the rocks on land, and to sail several times round the islands, boating 
along those parts of the cliffs which presented features of special geological 
importance. 
In the St. Kilda islets three groups of rock differing from each other in 
age may be recognized. 1st, A series of gabbros, dolerites and basalts which 
have been intruded through and between each other as sills ; 2nd, a mass of 
granophyre which invades these sills ; and 3rd, abundant dykes and veins of 
basalt which occur both in the basic and acid masses. 
From the extension of the basalt-dykes across the Outer Hebrides it is 
clear that the Tertiary volcanic region reached at least to within 60 miles 
of St. Kilda. Whether or not it stretched over the intervening space 
now overflowed by the Atlantic must be matter for conjecture. There 
can be no doubt that the intrusive rocks of St. Kilda are in age and 
origin the equivalents of those of the Inner Hebrides. The remnants 
left of them were assuredly not superficial extrusions, hut are charac- 
teristic examples of the more deep-seated intrusions of the Tertiary 
volcanic period. Down to the most minute details of structure they 
reproduce the features so well displayed by the gabbros and grano- 
phyres of Skye, Bum and Mull. If it is demonstrable in the case of 
these islands that the intrusions have taken place under a deep cover of 
basalt- sheets, now in large part removed, the inference may legitimately be 
. 1 Description of the Western Isles, vol. ii. p. 54. 
- In an article on the general geological features of the Outer Hebrides contributed to A 
Vertebrate Fauna of the Outer Hebrides, by .T. A. Harvie-Brown and T. E. Buckley, 1888. 
3 British Association Report, 1885, p. 1040, and a much fuller paper in the Proceedings of the 
Inverness Field Chib, vol. iii. (1884), p. 72. 
